The invention relates to optical element units for exposure processes and, in particular, to optical element units of microlithography systems. It also relates to optical exposure apparatuses comprising such an optical element unit. Furthermore, it relates to methods of supporting an optical element. The invention may be used in the context of photolithography processes for fabricating microelectronic devices, in particular semiconductor devices, or in the context of fabricating devices, such as masks or reticles, used during such photolithography processes.
Typically, the optical systems used in the context of fabricating microelectronic devices such as semiconductor devices comprise a plurality of optical elements, such as lenses and mirrors, gratings etc., in the light path of the optical system. As a part of an illumination system, such optical elements cooperate in an exposure process to project light provided by a light source onto a mask, reticle or the like. As a part of a projection system, such optical elements cooperate in an exposure process to transfer an image formed on the mask, reticle or the like onto a substrate such as a wafer. Said optical elements are usually combined in one or more functionally distinct optical element groups. These distinct optical element groups may be held by distinct optical element units.
Optical element groups comprising at least mainly refractive optical elements, such as lenses, mostly have a straight common axis of symmetry of the optical elements usually referred to as the optical axis. Moreover, the optical element units holding such optical element groups often have an elongated substantially tubular design due to which they are typically referred to as lens barrels.
Due to the ongoing miniaturization of semiconductor devices there is a permanent need for enhanced resolution of the optical systems used for fabricating those semiconductor devices. This need for enhanced resolution obviously pushes the need for an increased numerical aperture and increased imaging accuracy of the optical system. Furthermore, to reliably obtain high-quality semiconductor devices it is not only necessary to provide an optical system showing a high degree of imaging accuracy. It is also necessary to maintain such a high degree of accuracy throughout the entire exposure process and over the lifetime of the system.
One of the problems arising in the context of imaging accuracy is the introduction of stresses into the optical elements via the elements holding the respective optical element. Such stresses introduced into the optical element may have several origins. One origin is the mass distribution of the optical element itself and the location and distribution of the support elements supporting the optical element which leads to a certain mass induced stress distribution within the optical element.
Another origin of such stresses is a difference in the coefficient of thermal expansion between the optical element and its support structure. The difference in the thermal expansion behavior of the optical element and its support structure causes a shift between the optical element and its support structure. Depending on the rigidity of the connection between the optical element, this thermally induced shift is more or less hindered by the connection. A connection that is rigid in the direction of this thermally induced shift leads to an introduction of considerable stresses into the optical element. In the optical exposure systems mentioned above, this problem mainly arises in the radial direction of the optical element, i.e. in or parallel to the plane of main extension of the optical element.
To deal with this problem it is known, e.g. from U.S. Pat. No. 4,733,945 to Bacich, to provide a ring shaped holder for a lens contacting the lens via a plurality of connector elements. These connector elements are formed monolithically with the holder and adhesively bonded to the outer circumference of the lens. The connector elements comprise a leaf spring part that is compliant in the radial direction of the lens such that the stresses introduced into the lens upon thermally induced expansion are reduced. However, this solution has the disadvantage that, once the adhesive bonding of the lens and the holder is achieved, virtually no further correction or adjustment of the position of the lens relative to the holder is possible anymore at reasonably small effort.
A solution to this problem is known e.g. from U.S. Pat. No. 6,859,337 to Oshino et al. Here, the connector elements comprise two longitudinally aligned leaf spring elements connecting a first connector part attached to a lens to a second connector part connected to a holder ring. The leaf spring elements provide the radial compliance outlined above. The connector elements are not formed monolithically with the holder but removably mounted to the holder. However, this solution has the disadvantage that considerable effort is necessary when mounting the connector elements to the holder ring in order to avoid introduction of pre-stresses and deformations into the leaf spring elements, e.g. due to fastening torques, which may otherwise lead to the introduction of additional stresses into the optical element.
Such stresses introduced into the optical element may cause local anisotropies within the material of the optical element leading to so called stress induced birefringence effects. Among others, such birefringence alters polarization of the light which is of particular disadvantage when occurring in an illumination system.
In this context it has to be noted that the illumination system usually is subject to considerably larger temperature variations during operation of the exposure apparatus than this is the case for the projection system. This is due to the fact that, usually, no active cooling circuitry is provided for the illumination system. Thus, while the temperature deviation of the temperature from a setpoint value in the projection system is kept to be less than 1 K, the temperature deviation in the illumination system usually ranges up to 6 K, sometimes even up to 10 K.
In general, birefringence effects and deformations of the optical elements deteriorate the optical performance of the optical system. Thus, it is desirable to avoid or compensated for these to the highest possible extent.
One approach to avoid such adverse effects is to provide space between the optically used area of the optical element and the contact location of the support structure with the optical element that is sufficiently large for a reduction of the stresses introduced via the support structure. Such a solution is known e.g. from U.S. Pat. No. 6,867,848 B2 to Ebinuma et al. Here an intermediate support ring is adhesively connected to the lens. The intermediate support ring has a coefficient of thermal expansion that corresponds to the coefficient of thermal expansion of the lens. The intermediate support ring is connected to an outer support ring of different coefficient of thermal expansion via radially compliant connectors.
While such an intermediate support ring provides enough space for a reduction of the stresses introduced into it via the external support structure, it has the disadvantage that it is rather costly due to the necessary adaptation of its coefficient of thermal expansion. Furthermore, it introduced a further interface location and, thus, further positioning errors.